This invention relates to ink jet printing, and more particularly to techniques for preventing overheating.
Thermal ink jet printing normally seeks to offer high print quality and fast printing speeds. While these often face tradeoffs, with a reduction in one factor permitting an improvement on the other, development efforts seek ways to improve both, or at least to improve one without significant compromise to the other.
Print quality is often improved by increasing printing resolution, so that more printed dots per inch (DPI) are generated for a more detailed image. However, increasing the resolution means not only decreasing the spacing of nozzles on an ink jet print head orifice plate, but increasing the frequency at which the nozzles are actuated as the print head scans over a media sheet to generate a printed swath. While effective in some printing conditions, it has been found that high density printing at high frequencies causes print head overheating. This may render the printed output unusable, as the pen may cease to function. In severe cases, the pen may be damaged permanently.
To avoid overheating, printing frequency may be limited, either by limiting resolution along the scan direction (sacrificing print quality) or by limiting scan velocity (sacrificing speed). Another measure is to pause printing occasionally, such as at the end of each scan, allowing the print head to dissipate an adequate amount of heat energy accumulated during high frequency printing. This suffers the disadvantage that print quality may be sacrificed, as some printing modes and ink types require a xe2x80x9cwet edgexe2x80x9d of a prior printed swath as the next swath is laid down, to avoid visible knit lines at junctions between swaths. Another disadvantage is the time delay caused by the pausing, which reduces overall printing speed. Even if these were tolerable, pausing after each swath is unsuitable for larger format printers, which may be overheated even during a single swath. As printers are operated at higher resolutions and speeds, the issue of overheating on a single swath may arise even in smaller format printers.
Large format printers have addressed the issue of overheating during a single swath of high density printing by printing in an interlaced manner. That is, a swath is laid down in several passes, with only an integral fraction of the nozzles being used for each pass. For instance, the odd nozzles are used on the first pass, with the evens on the second to form the full swath. A three-pass mode uses every third nozzle on the first pass, then the set of nozzles offset by one from the first set on the second pass, then the remaining nozzles on the third pass. Another method is to create a checkerboard pattern and reduce the firing frequency of individual nozzles and still use the same number of nozzles to create each pass.
To avoid needless speed reduction while printing less dense portions of the printout, the printer may switch between interlaced mode and normal mode. This has the disadvantage of impairing print uniformity, as each mode may have a slightly different appearance. Also, the transitions between print modes may be complex, as printing often occurs with overlapping or shingled techniques that do not easily transition without complex software algorithms. In addition, the electronics required to store and analyze a page of print data to determine which modes are suited to which portions requires costly memory resources on the printer or connected computer, and the processing time also reduces printing speed. The transition in and out of these slower printmodes also has a speed penalty at the transition, extra sweeps may be required to complete one block and then start the next. Depending on the data content this can be severe.
A further disadvantage of the interlaced technique is that it provides compensation in often excessively large increments. This means that a slight density excess will lead to a speed penalty by a factor of two. Any density levels above 2.0 require a speed penalty by a factor of three, etc.
The present invention overcomes the limitations of the prior art by providing a printer and method of printing. The method includes receiving print data for a swath to be printed. The swath has a matrix of pixels arranged in rows and columns. For a succession of adjacent row segments of the swath, a cumulative total of pixels to be printed is calculated. If the cumulative total exceeds a preselected threshold upon inclusion of row segment, printing is limited to a limited subset of row segments of the swath.